Introducing fluids to patients, such as patients in shock, is often the first step in medical treatment. However, not all patients are responsive to fluid. For example, responsiveness to fluid administration may be based on the amount of fluid in the vascular space of the patient. Responsiveness to fluid administration may also be based on how full the heart is before each contraction, referred to as preload or ventricular preload. The relationship between the preload state and contractility of the heart has been described and has been shown graphically via the Starling curve.
For example, FIG. 1 illustrates a Starling curve 100 that depicts the relationship between the ventricular preload state of a patient and stroke volume. Referring to FIG. 1, when the preload state of the patient is relatively low, a small change in preload results in a relatively large increase in stroke volume, as illustrated at position A. In contrast, when the preload state of the patient is relatively high, a small change in preload results in a very small change in stroke volume, as illustrated in position B.
While a patient whose heart is in condition A may benefit from fluid administration, a patient in shock whose heart is in condition B may not benefit and may actually be harmed by fluid administration.
Respiratory variation in the pulse pressure of ventilated patients has been shown to correlate with fluid responsiveness. Current methods to monitor the pre-load sate of the heart and fluid responsiveness include echocardiography and the placement of a Swan-Ganz catheter. The Swan-Ganz catheter is inserted into the patient and is carefully moved to the heart to obtain pressure measurements from different vascular compartments. Thermal dilution is used with the Swan-Ganz catheter to measure cardiac output and stroke volume is calculated by dividing cardiac output by the pulse rate. Stroke volume can then be compared before and after fluid administration to determine the preload state of the heart. A problem with using a Swan-Ganz catheter is that the catheter is very invasive and is difficult to place in many patients, such as pediatric patients.
Other monitoring systems track cardiac output and can provide assessments of fluid responsiveness, but require a large central arterial catheter that places the extremity at risk and is unsuitable for use in many patients, such as children. In addition to being very invasive, current methods aimed at quantifying respiratory variation of the pulse pressure are not performed in a continuous manner that enable medical personnel to receive current information regarding the state of the patient.